File integrity checks in 10.0.2


I have an app which is still in testing that is using a 3rd party PHP library. Previously I use to install this into
/nextcloud_location/3rdparty using composer but now with 10.0.2 doing what look like more extensive integrity checks
I can’t install this library into that location. I had a vague recollection that it might be installable into
/nextcloud_location/apps/appname/3rdparty using composer which I’ve done. However the app PHP code
isn’t finding the library. What do I need to do, so the app picks up the library?